A typical digital camera focuses an optical image of a scene onto an image sensor, which samples the optical image to generate an electronic representation of the scene. The electronic representation is then processed and stored as a digital photograph. A conventional image sensor is configured to generate a two-dimensional array of color pixels from the optical image. Each color pixel typically includes an independent intensity value for standard red, green, and blue wavelengths. A properly generated digital photograph will have a natural appearance, resembling direct observation of the scene by a human observer. To generate digital photographs having a natural appearance, digital cameras attempt to mimic certain aspects of human visual perception.
One aspect of human visual perception that digital cameras mimic is dynamic adjustment to scene intensity. The human eye is able to adjust to a wide range of light intensity in the same scene. Digital cameras dynamically adjust to scene intensity by selecting a shutter speed, sampling sensitivity (“ISO” sensitivity associated with sensor sensitivity), and aperture to yield a good overall exposure level when generating the digital photograph. However, for a given exposure setting, a typical scene may include areas spanning a dynamic range that exceeds the dynamic range of a conventional image sensor, leading to overexposure, underexposure, or a combination of both in the same scene. The scene may also include important visual detail at intensities that are poorly quantized by the image sensor when configured for a specific exposure level, leading to quantization error, which appears as “banding” or unwanted “posterization.”
Techniques known in the art as high dynamic range (HDR) photography provide for sampling and representing image information having a high dynamic range substantially representative of dynamic range within a given scene. HDR photography conventionally involves sampling a set of digital photographs, referred to as an image stack, at different exposure levels for the same scene to capture image detail at different dynamic range levels. Images comprising an image stack may be combined to synthesize a single digital photograph that represents contrast and image detail depicting the full dynamic range of the scene. In certain scenarios, the full dynamic range is mapped to a reduced dynamic range for display on a conventional display device, such as a liquid crystal display (LCD). The digital photographs comprising the image stack are assumed to contain substantially consistent content that is sampled at different exposures. The digital photographs are conventionally sampled sequentially, with an inter-sample time separating the capture of each digital photograph.
During sequential sampling, the digital camera may move, such as from hand motion or vibration. During sequential sampling, the scene may also change, such as from people, animals, or objects moving in the scene. As a consequence of such motion or change, each digital photograph within the set of digital photographs needs to be aligned to the other digital photographs to provide spatial consistency prior to a combination operation. As inter-sample time increases, the likelihood of uncorrectable misalignment among the digital photographs increases, as does the likelihood of uncorrectable divergent content within the digital photographs. Examples of divergent content include birds flying in a landscape scene, and people talking or otherwise moving in a social scene. A common example of uncorrectable divergent content arises when a person is moving their head, such that the first digital photograph in the set of digital photographs captures the person's face, while a second digital photograph captures the side of the person's head. Conventional alignment techniques are computationally intense and still cannot adequately correct for content changes, such as aligning a face with the side of a head. Furthermore, conventional image capture techniques typically require significant inter-sample time, which may be greater than a thirtieth of one second. Significant inter-sample time commonly leads to capturing uncorrectable divergent content or digital photographs that cannot be properly aligned within the image stack, thereby visibly and negatively impacting the quality of synthetic digital photographs generated from the image stack.
As the foregoing illustrates, there is a need for addressing this and/or other related issues associated with the prior art.